Improve a bit of dataclasses support, so at least the attributes can be seen

see #1213

test/completion/decorators.py

@@ -310,11 +310,14 @@ follow_statement(1)
 # class decorators should just be ignored
 @should_ignore
 class A():
+    x = 3
     def ret(self):
         return 1
 
 #? int()
 A().ret()
+#? int()
+A().x
 
 
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